http://dl.dropbox.com/u/2769457/ad623.png
what is the range of voltage difference across the thermocouple?
Why you are not using AD595?
When you are using opamp you should care about its input and output offsets and drifts VS ambient temperature.
After that you should compensate reference (or cold) junction temperature in your PIC.
The AD595 do all this works! The AD595 have an amplified and compensated output that you can use in your circuit.
Which compiler you are using?
I think that "Short int" is an 8bit integer while you used it for holding a 10bit ADC result.
---------- Post added at 09:40 ---------- Previous post was at 09:32 ----------
Zwilorg!
Note that AD595 amplifies and compensates the thermocouple output but does not correct the nonlinearity of thermocouple output!
It's better to first measuring the output voltage of AD595 and then convert it to temperature by using type K "VT" table.
Another way is to convert measured voltage to temperature by using formula.
---------- Post added at 09:54 ---------- Previous post was at 09:46 ----------
You can test your circuit by using a voltmeter. in 25C the output of AD595 should be about 250mv.
Also for more information about thermocouples, VT tables and conversion coefficient see "www.srdata.nist.gov"
If you get a 250mv (in 25C) output from AD595, then every things is OK!
Try this:
unsigned int adc_res;
adc_res = (ADRESH<<8) + ADRESL;
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